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Substitutes for the WD-11

Westinghouse introduced the WD-11 tube in 1922 for
their Aeriola, Sr. (Model RF). They needed a tube
which could be heated with dry cells instead of
storage batteries. The WD-11 was also used in the RS,
Regenoflex, Radiola X, Radiola III and Radiola III-A,
sets made for RCA by Westinghouse. The development of
this tube is well documented [1].

General Electric developed
the UV199 tube independently, with the same objective of
economical dry cell operation. The UV199 was introduced in
1923 with the Radiola IV. Its characteristics were similar
to the WD-11 except for the filament demand of 3.3V/.06A
vs. 1.1V/.25A for the WD-11. Besides its filament economy,
the UV199 was also more robust and less microphonic.

The filament structure of the
WD-11 is notoriously delicate. When a filament burns out,
it may flop against the plate applying B+ to the filament
circuit with disastrous results [2]. RCA chose the UV or
UX199 for subsequent superheterodyne and regenerative
receivers. No apparatus using the WD-11 was designed after
1924, and it became obsolete so quickly that RCA issued
Service Bulletin #6 (Dec. 11, 1925) showing how to convert
the Radiolas III and III-A to UX199 and UX120 tubes [3].

Westinghouse issued the
WD-12, a UV-based version of the WD-11, in 1923. It was
replaced by the UX-based WX-12 in 1925 for converting
storage battery sets to dry cell operation. The Crosley
"Pup" appears to be the only radio factory-designed for
the WD- or WX-12. The WD-11 and WX-12 were dropped from
the official RCA product line in 1933 [4].

Those who want to play these
early sets have a tube problem. RCA quit stocking WD-11
tubes 70 years ago. Today they are prohibitively
expensive, being worth more than the radios using them.
Because of their value, those who have WD-11s are
reluctant to put them into service.

UX199s can be used per the
RCA bulletin. Adapters are still around, but both are
quite expensive. The 864 (VT-24), developed for aircraft
use, is a popular substitute because it has the same
filament requirements and similar characteristics, but is
also now quite expensive.

The Type 30 has been used as
a substitute, but is both expensive and requires an
oddball filament voltage. Another substitute is the Loctal
3D6, a beam pentode, connected as a triode and used with
an adapter. Its filament requirements are similar to the
WD-11. Miniature pentodes (1L4, 1S4, 3S4, 3Q4, 3V4) used
in the same manner have also been described [5].

Another elegant solution is
to use a miniature, wire-lead 5676 in a WD-11 base. A
silvered test tube can be fastened over it over it to give
the appearance of a WD-11.

Testing Possible
Substitutes

I decided to carry out some
tests on possible substitutes that would be cheap,
plentiful, functional and resemble the WD-11. A look at
the tube manual uncovered the following possibilities:

The first four tubes are
shown for comparison purposes only. Types 1G4, 30 and 864
are not good candidates due to their high cost. The last
three turned out to be good possibilities. Characteristics
of the triode-connected 1Q5, 1A5 and 1LA4 are not listed
in tube manuals and had to be measured. The 1Q5 is a beam
power tube and the 1A5 a power pentode. The 1LA4 is a 1A5
with a Loctal base. The characteristics of the other tubes
were taken from the tube manual.

RCA recommends that the
Radiola III-A be operated with 22.5V on the detector and
90V with -4.5V bias on the three amplifier tubes.
Accordingly, the transfer characteristics of the WD-11,
864 and the possible substitutes were examined at Ep =
90V.

Fig. 1 shows the results. The
WD-11, 1Q5 and 1A5 have long, linear transfer curves. The
864 and 1LE3 have a much narrower operating range. The
steeper slope of the 1Q5 and 1A5/1LA4 reflect their higher
transconductance. All the tubes have similar amplification
factors (µ) except for the 1LE3 which is nearly twice that
of the others.

Making a WD-11 Base
Template

Because of the unique WD-11
base, substitutes must either be re-based or provided with
adapters. Re-basing yields good looking results, but when
it's time to replace the tube, you have to do everything
all over again. Adapters are easier to make and use and
involve less risk to the tubes. When the tube is worn out,
just plug in a new one. There are some adapters sold
commercially, but they are expensive.

You can make your own
adapters at very little cost using old 4- or 5-pin bases
from dud tubes. These should be the small, 1-1/8" diameter
bases. It is easier to drill new holes in 4-pin bases, but
5-pin bases can also be used. A template will simplify
your work.

The WD-11 pin layout is shown
in Fig. 2. The template was made from a scrap of 1/4"
thick Plexiglas about 2" square from the local glass shop
and a similar piece of Bakelite from an old radio panel.
Lay out the pin locations on the Plexiglas as shown. Use a
sharp scribe and be accurate. Drill one 3/16" hole and
three 1/8" holes where indicated. If you spot the holes
first with a center drill, the main drill will run true. A
drill press is essential for precision. Also drill a 1/8"
hole at the center of the pin circle for later use.

Now, clamp the Plexiglas and
Bakelite pieces together and drill two holes near opposite
corners for bolts to hold the two parts together. Bolt the
parts together and, using the center hole as a guide,
drill through the Bakelite with the 1/8" drill.

This hole in the Bakelite is
the pilot hole for a 1-1/8" hole saw which you will next
use to make a hole in the Bakelite to fit the tube base to
be adapted. This hole may need a little filing or sanding
to get a snug fit. Fig. 3 shows the template and Bakelite
bolted together.

Making a WD-11 Base

Remove each of the existing pins on your tube base by
holding it with pliers so it can't rotate and drilling
away the flange from the inside. When the flange is
removed, the pin will pull out easily. Use a drill of
the correct diameter to remove the flange without
removing any Bakelite. Scrape all the old cement out
of the base. Insert the base into the template so the
bottom is against the Plexiglas. You can now easily
see how to position the base so that the new holes
will lie between the old holes.

Drill a 3/16" hole in the
base using the hole in the template to guide it. Insert a
small piece of 3/16" rod through both template and base to
prevent slippage and drill the three 1/8" holes and one
3/16" hole. The new pins are made from thin-wall brass
tubing in 3/16" and 1/8" diameters from a hobby shop. This
tubing is full diameter, while the holes will be slightly
undersize, therefore the new pins will fit tightly and
need no anchoring.

Cut the tubing to 1-3/4"
length. I use a miniature tubing cutter from the hardware
store. It gives a rolled edge at the cut which makes
insertion and soldering easy. The best way to insert the
pins in the base is to chuck them in your drill press and
use the press to push them in. Make two 9/16-long collars
to use as depth gauges. Drill one to slip over the 3/16"
pin and one to slip over the 1/8" pins. I made these from
tubular spacers.

Start the brass tubing into
the base by lightly tapping it with a small hammer. Then
slip on the collar and press the tubing into the base
until its end is flush with the end of the collar. Watch
it closely to be sure it goes in straight. Now your pin
will protrude the correct distance from the base.

Fabricating Adapters

The sockets I used to make
adapters for the substitute tubes came from the junk box.
They were the molded Bakelite kind with molded-in mounting
plates. The diameter below the plate is about one inch so
they fit snugly into the bases with just a little filing.
Cut off the mounting ears with tin snips, but leave the
rest of the mounting plate in place. Wiring of the adapter
will depend on the type of tube you are using as a
substitute. Diagrams for the 1LA4/1LE3 and 1A5/1Q5 are
shown in Fig. 4. The resistor across the filament
connections needs explanation.

Volume in these old sets is
controlled by varying the filament voltage through a
rheostat. It is sized for use with 0.25A tubes. Because
the substitutes draw only 0.05 to 0.1A, the rheostat will
have little effect on filament voltage and the set will
run wide open with no volume control action.

For substitutes to behave
like WD-11s, they must draw similar filament currents. The
1/4-watt resistor, R, shunting the filament increases the
total circuit current to about that of a WD-11. This will
give good volume control action. R should be 6.8 ohm for
the 1Q5 and 5.1 ohm for all other tubes.

Connect the resistor between
the filament lugs on the socket and solder wires to them
and to the grid and plate lugs. The wires can be bare
except the one from the plate (or grid depending on how
you orient the socket with respect to the base). This wire
needs to be insulated where it crosses over the socket.
Guide the wires through the pins in your WD-11 base, seat
the socket firmly in the base and solder the wires to the
pins.

Drill two small holes on
opposite sides of the base into the edge of the socket.
Make sure that the holes go between socket pins. The
diameter of the hole will depend on the size of the
self-tapping screws you use to fasten the socket to the
base. It is a good idea to experiment on some scrap
Bakelite before drilling the sockets.

I use #0 x 1/4" or #1 x 1/4"
screws. Using a larger drill, enlarge the hole in the base
shell so that the screw passes freely. Do not drill into
the hole in the socket edge. Insert the screws to hold the
base and socket together. Don't over tighten them or you
may crack the shell.

Fig. 5 shows (A) a WD-11
tube, (B) a re-based 864, (C) a 1LE3 in its adapter, and
(D) a 1A5 in its adapter. The superficial resemblance of
the substitutes to the WD-11 is obvious. Details of the
adapter construction are also visible.

Testing the Substitutes

A Radiola III-A was equipped
with WD-11 tubes and its operation studied. This set has 2
sections: a regenerative detector and an AF amplifier
followed by a push-pull second audio output stage. There
is provision for headphones after the first audio stage
and each section has its own filament rheostat.

Because of the various
interacting adjustments to be made for proper reception,
there could be no "standard" setting for comparing tubes.
so the test was subjective. In each case, a weak station
was tuned in and the positions of the filament and
regeneration controls noted at the settings for loudest
volume and best sound quality. The position of the
regeneration control at which oscillation occurred was
also noted.

The set was operated with
22.5V on the detector and 90V on the audio amplifiers with
-4.5V bias. The filament supply was 1.5V. A voltmeter was
connected across the filament pins so as not to exceed
1.1V for the WD-11s. At first, only the "front" half of
the set was operated (detector + first audio) with
headphones.

Using WD-11s, the set was
adjusted for best volume and sound. Then the WD-11s were
replaced first with 1LE3s, then with 1A5s, and the set
re-adjusted for optimum reception in each case. You'll
recall that the 1LA4 is electronically identical with the
1A5.

The 1LE3s were rather touchy.
Oscillation onset occurred at a much lower setting of the
regeneration control and the filament control had to be
reduced to avoid distortion. Nevertheless, performance was
satisfactory and quality was acceptable. The volume was
louder because of the higher µ of the 1LE3.

The 1A5s also oscillated at a
lower setting than WD-11s and required reduction of the
filament voltage, but overall performance was much
smoother than the 1LE3 and distortion was less at high
volume.

A horn speaker was then
connected to the second push-pull audio stage and the
filament rheostat turned on. There was a noticeable
tendency for the 1LE3s to distort at high volume levels,
but satisfactory room-level sound could be achieved with
filament adjustment. As before, the 1A5s were smoother and
more like WD-11s in operation. Distortion was less;
room-level sound was easily achieved. In all cases, volume
control action was as good as with WD-11s.

After similar tests were done
with 1Q5s, it was found that there was little perceptible
difference in the performance of 1A5s, 1Q5s and 1LA4s. In
conclusion, I prefer the 1A5 or 1Q5 for a substitute. They
are cheap and available and perform very much like the
WD-11. The 1LA4 works just as well, but is a bit more
expensive. If you happen to have some 1LE3s on hand,
however, they are also functional and will do the job.